Present invention relates to a ductile cast iron roll which has excellent resistance to cracking and a method manufacturing said roll.
In the rough rolling stand of the hot rolling machine, the thermal load on the roll is extremely heavy due to the high temperature of the material to be rolled and due to slow rolling speed. For this reason, the roll for the rough rolling is apt to crack due to the thermal fatigue, and as a result, there are many break downs caused by the development of these cracks in the roll.
Hitherto, ductile cast iron rolls were used in the rough rolling stand of the hot rolling machine; these prior art rolls have globular graphites dispersed there; the said dispersed globular graphites block the development of the cracks and thereby the cracks are dispersed like the pattern on a tortoise shell; therefore, ductile cast iron roll is believed to have been provided with a comparatively good resistance to cracking.
However, conventional ductile cast iron roll was still insufficient in the cracking resistance for severe conditions of heat and load when the roll is in the rough rolling stand of the hot rolling operation as mentioned above, and the occurrence of the breaking accident of the roll was not small in number. It is needless to say that the occurrence of the breaking accident of the roll will cause not only the increase in the consumption volume of the roll but also the decline in the rolling work efficiencies and resultant lowering of the productivity.
In order to give an excellent resistance to breaking to the roll, it is necessary to enhance the resistance to the occurrence and development of the cracks, namely the resistance to cracking must be enhanced. In order to enhance this cracking resistivity, it is necessary to improve the toughness, namely the elongation value at the tension test and the flexibility volume at the breaking test must be improved. For this purpose there are two ways, one is to decrease the volume of cementite in the structure and another is to enhance the toughness of the base. As to the former method, though it was put in practice up to this time, the decreased volume of cementite in the structure is apt to cause a decline in the abrasive resistivity as a result. Because the roll for the rolling machine which is the object of the present invention requires considerable abrasion resistivity, there is naturally a limit to the improvement of the toughness by this method.
Therefore, the present invention focused on the latter method, and after making various studies, it became possible to specify the chemical compounds and compositions, and structures as mentioned above, thereby the resistance to the occurrence of heat cracking is enhanced and, the resistance to breaking made superior.